Function reserve_mesh_reference_if_active 

pub fn reserve_mesh_reference_if_active(handle: Py<PyShared>) -> bool

Reserve a slot for a pending mesh whose *MeshRef is not available yet, registering handle for a sender-side fill once the mesh resolves.

Returns true if a slot was reserved (the caller emits a pop_mesh_reference placeholder); false otherwise (the caller keeps its current behavior).